The present invention relates to a lithographic printing or more particularly to a lithographic plate cleaner. Lithographic plates used in the graphic arts have image areas which are oleophilic and non-image areas which are hydrophilic which maintain the definition of the ink when the plate is used in the press to produce a clear image. After repeated use of the plate and aging of the surface, the non-image areas of the plate are less able to reject ink and may tend to retain some ink. This is called scumming. The image areas of the plate may also become less oleophilic and hence less able to hold the ink. This may result in what is known as image blinding.
This results in poor quality printing and the lithographer must then expend a good deal of effort to restore the plate to a useful condition.
Cleansing agents hitherto used consist of dispersions and contain a number of heterogeneous substances such as, for example, in the dispersion described in U.S. Pat. No. 2,780,168, which serves as cleansing agent for greasy printing plates and contains, in the aqueous phase, monofunctional and multifunctional alcohols and a colloidal dispersion of silicic acid and, in its non-aqueous phase, preferably petroleum ether.
Various other compositions have also been used heretofore as cleansing solutions. For example, U.S. Pat. No. 3,108,535 employs various phosphonic acids. U.S. Pat. No. 3,289,577 uses various sulfonic acids. U.S. Pat. No. 3,060,848 describes a plate cleaner having significant amounts of aromatic solvents. Likewise U.S. Pat. No. 4,162,920 employs substantial amounts of aromatic solvents. Plate cleaners are also described in U.S. Pat. Nos. 2,780,168 and 3,679,479.
It has always been desirable to provide a plate cleaner which remains a stable emulsion during use, maintains its viscosity, is capable of substantially restoring the image and non-image plate areas to a useful discriminatory state and yet does not contain a substantial amount of components, such as aromatic solvents, which are harmful both to the user and to the environment. The present invention substantially advances the art toward the achievement of these goals.